The present invention relates to an ion source used in an ion implanter of a type used to deposit impurities into semiconductor integrated circuits at a high energy of acceleration.
Industries such as the semiconductor manufacturing industry must have ion implanters which are capable of accelerating ions to various energies and directing the accelerated particles onto a semiconductor substrate so as to implant such particles to a desired depth. Such implanters must be capable of generating a gas or vapour of desired particles and then ionizing that gas or vapour so that the resultant particles can be accelerated under the influence of an electric field. The method of producing ions consists of first vapourizing the molecules usually by heating the substance to a desired temperature and then directing the particles to an arc chamber which bombards the neutral particles with electrons to ionize them. The apparatus for vapourizing the substance consists of a cylindrical crucible, usually graphite, which is electrically heated in a high vacuum by passing a current through a resistive wire such as tungsten wrapped around an electrically non-conducting container enclosing the crucible. The substance to be vapourized is placed inside the cylindrical crucible which has a hollow stainless steel tube down its center and heated until its vapour is released. The crucible assembly is at an angle of about 30 degrees. Released vapour enters the stainless steel tube in the interior of the crucible and is allowed to escape out an end of the tube and into an arc chamber.
Commercially available ion sources are generally acceptable for such substances as arsenic and phosphorus which go directly into a vapour phase but are not acceptable for substances such as antimony which go through a liquid phase before releasing vapour. Current crucible designs allow liquid antimony to resolidify in the crucible on the stainless steel vapor feed tube eventually blocking the release of antimony vapor. This condensation occurs primarily after shutting down the apparatus and allowing it to cool. Since the crucibles used to house the substance to be vaporized are expensive and more importantly since changing a crucible involves expensive production line down time, current ion sources are inadequate for such materials. One possible solution is the use of an antimony source material in gaseous form. Unfortunately, such gas presents severe safety problems due to its high toxicity.
Accordingly, it is an object of the invention to provide an improved ion source for an ion implanter. It is a further object to provide an ion source capable of generating antimony vapour from solid or liquid antimony.